In the automotive industry, certain components of the suspension system need to be capable of withstanding repeated stresses during their lifetime. When designing such a component, various treatments are specified for increasing its ability to withstand fatigue. In order to evaluate the effectiveness of such treatments and consequently in order to evaluate the lifetime of the component, standardized stressing cycles are defined which consist in applying a succession of deformations to said components until it breaks. At present, it is desired to be able to guarantee that no breakage can appear under 100,000 cycles. The calculations performed in developing such components show that certain portions of the components are more likely to break than others, and in particular the bends. It is thus mainly these portions which need to be treated in order to guarantee the ability to withstand at least 100,000 cycles. For example, the ability to withstand fatigue of an anti-roll bar is evaluated by subjecting the two ends of the bar to opposing displacement cycles and counting the number of cycles up to breakage. Analogous tests can be defined for evaluating the ability to withstand fatigue of an active half-bar or of any other element of comparable shape. The problem of determining ability to withstand fatigue with such elements is becoming ever more critical, mainly because of two trends:                vehicles are becoming heavier because of the increasing amount of on-board electronic equipment and increasing engine size; and        manufacturers are tending to define stabilizing bars of longitudinal axis that is offset very little from axes passing through the centers of the wheels; under such conditions, stabilizing bars are shorter and for a given wheel travel, the amplitude of the displacements of the ends of the bar increases as do the stress levels to which the bar is subjected.        
In addition, for 4-wheel drive vehicles, the amount of deflection at the ends of the stabilizing bar is even greater because of the desire to enable the vehicle to negotiate obstacles.
Also, weight reduction due to the use of a tubular bar leads (compared to a solid bar and for identical amplitude of movement) to an increase of strain.
One of the treatments for increasing the resistance to fatigue of a stabilizing bar or an analogous component consists in applying thermal quenching thereto. Depending on the methods used, such quenching is performed either after or before the bar has been shaped, i.e. after or before bending operations. Several techniques are known.
A first method consists in applying austenizing treatment to the bar in an oven, then in bending it while hot and while taking care to ensure that the temperature of the bar remains high enough for the steel to remain in the austenitic range. The bar is then quenched as a whole by being immersed in a liquid. It is accepted that this method of hot-bending followed by overall quenching does not enable highly-stressed bars to be produced. It is applicable only to making solid stabilizer bars.
Another method consists in cold-bending a solid bar, in heating it by conduction by placing its two ends between the electrodes of conduction heater equipment, and then in quenching. Quenching the bar leads to a martensitic transformation to the core of the bar, providing the steel was caused to be completely austenitic during the conduction heating stage. That method is slow and expensive.
Several attempts have been made to perform cold-bending followed by quenching that consist in causing an inductor and a shower to move along the component. Such a method has been found to be extremely slow and difficult to perform because of the electrical losses that occur in the flexible connection powering the moving inductor.
Hollow anti-roll bars are also known, i.e. bars made from a tube, and preferably of varying wall thickness so as to have greater thickness in the bends or other regions that are subjected to high levels of stress. The bar is bent while cold and is subsequently subjected to austenizing treatment in an oven having a controlled atmosphere, followed by quenching. The tubular bar is subsequently subjected to annealing treatment for one hour at 200° C. and it is then shot-blasted internally, the shot being injected by means of a nozzle. In that method, the bar is quenched throughout, thereby making it more difficult to forge its ends since forging then needs to be performed hot.